The present invention relates to a corrugated pipe, a method of forming the corrugated pipe and an apparatus for manufacturing the same, which is used for arranging therein cables such as power lines, communication lines and so forth.
As a method of forming the corrugated pipe of this type, there has been proposed such method as disclosed in, for instance, Japanese Unexamined Patent Publication No. 141387/1987 in which the central portion of a strip-like metallic material having both surfaces coated with a synthetic resin layer is bent along its axial direction to thereby form a substantially U-shaped recess in cross section and the strip material is bent in a spiral form followed by connecting the adjoining edges of the spirally wound strip material to thereby form a corrugated pipe. In the above-mentioned method, however, it was difficult to obtain a strong connection between the adjoining edges of the spirally wound strip material since the adjoining edges were merely joined by making both edges contact to each other or by overlapping the edges one on another followed by melting the synthetic resin layer covering the front and rear surfaces of the strip-like metallic material. Accordingly, when a force is applied to the corrugated pipe in its axial direction or a bending force is applied to the corrugated pipe, it is easily broken at a joined portion.
In the above-mentioned publication, there is also proposed such a method that the opposing edges of the spirally wound strip material are bent in a form of hook so that they are usually engaged. However, the operation for engaging the hooked edges is troublesome, and the process for forming the hooked edges is complicated.
On the other hand, in the case that the strip-like metallic material having both surfaces coated with a synthetic resin layer is bent in a spiral form, if the synthetic resin layer is made thick, the synthetic resin layer is apt to peel off from the surface of the strip-like metallic material or it is difficult to bend the strip material. Accordingly, it has been necessary that the synthetic resin layer coating the corrugated pipe obtained by a conventional method is made thin, which has poor corrosion resistance property. Accordingly, the outer surface of the corrugated pipe is easily corroded by undergrand water when the corrugated pipe is arranged in the ground.